Tritium location marker.
Tritium is hydrogen with two extra neutrons. That is, the nucleus is one proton and two neutrons instead of just a plain proton like normal hydrogen. Deuterium (see below) is hydrogen with only one extra neutron. Tritium is radioactive, while deuterium and hydrogen are not.
Tritium has two main uses: Thermonuclear weapons and glow in the dark key chains, buttons and exit signs. This particular item is a "location marker" used typically in military situations on ships to make visible the location of obstacles, etc. Tritium is also used as a tracer in certain biochemical reactions, because it can stand in for hydrogen, which is in everything, and you can determine where even vanishingly small quantities of it are by measuring the radioactivity.
Tritium key chains are banned in the US as a "frivolous" use of tritium, but you can legally buy location markers and such like provided you certify that they will be permanently affixed in a stationary location, which is registered with the government. My Periodic Table is such a registered location now, and the marker you see here is screwed down with three spanner-head tamper-proof screws per manufacturers recommendation. (Those frivolous Europeans do not have such limitations, and you can freely buy tritium key chains, tritium fishing lures, probably tritium bellybutton rings if you look hard enough.)
This thing, amazingly, will glow like this for the next twenty years with no batteries, no recharging with room light, no nothing, it just glows. The tritium, with a half-life of 12.5 years, decays emitting an electron, which strikes a phosphor coating inside the glass tube, which in turn emits the green light you see.
It's plainly visible even during the day, if you're in a shadowed corner of the room or cup your hands over it. In the dark it is quite bright.
I was looking at it recently and it's an awe inspiring sight, when you think about how many billions of atoms are decaying every single second to make the billions of photons you see, and that this will keep up every second of every minute for years and decades to come, and it still won't be all out of tritium. And that's in a tiny tube no bigger than the end of your thumb. It really gives you an impression of just how big the number of atoms is, and how small they must be.
While makers of these things claim no radioactivity escapes through the glass, this is only mostly true. I measure about 500 counts per minute, slightly but distinctly above background. Of course, this is about the same level I measure from 4 tablespoons full of salt substitute which people actually buy at the grocery store and then eat (it contains a small percentage of naturally occurring radioactive potassium-40). And it is stopped by a few inches of air. In other words, if you're concerned about it, don't enter the grocery store, because you'll get more there than from this marker.Source:AmerigloContributor:Theodore GrayAcquired:30 August, 2002Text Updated:29 March, 2009Price: $65Size: 1"Purity: >90%